SDAA249 January   2026 AMC3330-Q1 , INA148-Q1 , TPS61170-Q1 , TPSI2240-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
    1. 1.1 Background
    2. 1.2 Operation Principle
  5. 2Hardware Design
    1. 2.1 Solid State Relay (SSR)
    2. 2.2 Resistors
      1. 2.2.1 Minimum Resistance
      2. 2.2.2 Maximum Resistance
      3. 2.2.3 Resistance Ratio
    3. 2.3 Bias Supply
    4. 2.4 Amplifier
      1. 2.4.1 MCU on the Low Voltage Side
      2. 2.4.2 MCU on the AC Side
  6. 3Software Design
    1. 3.1 Settling Time
    2. 3.2 SSR Sequence
    3. 3.3 Voltage Threshold
    4. 3.4 Moving Average
    5. 3.5 Plausibility Check
    6. 3.6 Control Scheme Summary
  7. 4Simulation Results
    1. 4.1 Settling Time
    2. 4.2 Input Voltage Range
    3. 4.3 Plausibility Check
    4. 4.4 Accuracy
    5. 4.5 Influence of the Y Capacitor
  8. 5Summary
  9. 6References

3.3 Voltage Threshold

Based on the parameters in Table 2-2, assuming VDC is 40V, insulation resistance in a normal state is 10MΩ and in an insulation failure state is 110kΩ. Substituting VDC into formula Equation 18 and Equation 20, the input voltage under a normal state and N insulation failure state are:

Equation 24. V i n D C N O R ≅ 2 × R i n R s t + R i s o N O R × V D C = 0.052 V
Equation 25. V i n D C F L T ≅ R i n 0.5 × R s t + R i s o F L T × V D C = 0.33 V

Considering the effect of the Y capacitor, when the settling time is three times the time constant, there is a 5% error in voltage sensing, so the voltage threshold is set to 0.34V. Since the output of the operational amplifier has a voltage offset of 1.46V, the voltage threshold set in the MCU is meant to be 1.8V.